Recent technical advances have enabled the manipulation of small volumes of fluids, often in discrete droplets. Many of these systems utilize the phenomena of electrowetting to manipulate the small electrical droplets. Electrowetting on dielectric (EWOD) is the reduction in apparent contact angle of a fluid droplet using the capacitance of a dielectric layer which lies between the droplet and an electrode. Applying voltage across the barrier, between the droplet and electrode, causes charge migration to occur within the droplet and within the electrode which modifies the apparent surface energy of the droplet, causing its apparent contact angle to be reduced.
Applications for electrowetting, in general, are diverse, ranging from the shaping of micro lenses, fiber optics switching, display technology, and optical filters, to the creation of small low-power-consumption motors. When looking at applications for EWOD droplet transport, perhaps the most significant is lab-on-a-chip designs. Additionally, the use of droplet motion to assist in assembly of nano- and micro-scale components for microdevices holds promise.
Droplet movement by spatially asymmetric electrowetting, wherein only a portion of the droplet has its contact angle reduced resulting in the droplet motion, is known in the prior art. The prior art uses successive activation of discrete small electrodes, several of which are covered by the droplet. As each electrode is activated and the contact angle above it is reduced, the droplet is “handed off” from one electrode to another electrode. This hand-off accomplishes droplet movement in successive discrete steps, each step having a magnitude equivalent to the size of the electrodes. These prior art electrowetting systems require complex control systems to control the activation of the electrodes in the proper sequence and exhibiting the proper timing. Moreover, because electrowetting behavior is related to the voltage squared, the behavior typically shows no significant dependence upon the polarity of the voltage applied to the electrodes.
Accordingly, what is needed in the art is an electrowetting system that provides a substantially smooth, continuous, movement of a droplet across a single circuit that does not require complex control systems to accomplish movement of the droplet along a surface. What is also needed in the art is an electrowetting system that is dependent on the polarity of the applied voltage, such that the actuation direction of the droplet can be changed by changing the polarity of the applied voltage.
However, in view of the prior art considered as a whole at the time the present invention was made, it was not obvious to those of ordinary skill in the art how the limitations of the art could be overcome.